Scanning electron microscopy of freeze-dried preparations: relationship of morphology to freeze-drying parameters

The structure of the freeze-dried plug of several biological preparations was examined using scanning electron microscopy (SEM). A variety of carbohydrate preparations were frozen at two different cooling rates of 150 degrees C per minute and 1 degree C per minute. After lyophilization, residual moisture analysis demonstrated that under conditions of rapid freezing a drier product could be obtained. SEM observations showed the lyophilised plug structure of rapidly frozen material consisted of a fine amorphous meshwork, while material frozen slowly consisted of a leafy amorphous material. Freeze-dried mannitol preparations were shown to be of a crystalline nature. In most instances, rapidly frozen material was easier to reconstitute after lyophilization. SEM of proteinaceous preparations which had been frozen at differing rates also showed a variety of different appearances which were probably related to the direction of ice growth. During ice growth formation the migration of buffers and salts towards the surface may occur. Localised high salt/buffer areas may cause deterioration of the product and reduce shelf life. SEM observations were made on the freeze-dried plug of preparations which had collapsed during the lyophilization process. Typically, the basal region of the plug had a more solid structure than the open meshwork of successfully dried material.